Polishing method, polishing device, glass substrate for magnetic recording medium, and magnetic recording medium

ABSTRACT

There are disclosed a polishing method and a polishing device in which cleaning of a glass substrate surface can be achieved to a high level. A glass substrate (MD substrate  1 ) in the shape of a circular disc having a circular hole in a center portion is immersed in an abrasive liquid  50  containing free abrasive grains, and an inner peripheral end surface of the glass substrate is polished using the abrasive liquid containing the free abrasive grains by rotating a rotary brush  4  or a polishing pad in contact with the inner peripheral end surface.

REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 09/162,056, filed Sep. 28,1998, now U.S. Pat. No. 6,280,294.

This application claims the Paris convention priority right of JapanesePatent Application No. Hei 9-282613 filed on Sep. 30, 1997, the entiredisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

(i) Field of the Invention

The present invention relates to a polishing method and a polishingdevice, particularly to a polishing method and a polishing device whichcan preferably be used for polishing an inner peripheral end surface oranother surface of a glass substrate for a magnetic recording medium,and the like.

(ii) Description of the Related Art

An aluminum substrate has been broadly used as a substrate for amagnetic disc or another magnetic recording medium, but accompanyingminiaturization/thinning of the magnetic disc and densification ofrecording, the aluminum substrate is increasingly replaced with a glasssubstrate which is superior to the aluminum substrate in surfaceflatness and substrate strength.

In general, a glass substrate chemically reinforced to raise thesubstrate strength, or a crystallized glass substrate whose strength israised by crystallization is used as the glass substrate for themagnetic recording medium.

Moreover, for a magnetic head, high-density recording brings about atransition from a thin-film head to a magnetic resisting head (amagnetro-resistive head) (MR head) and a giant magnetic resisting head(a giant magnetro-resistive head) (GMR head). Therefore, it is expectedthat regeneration of the magnetic recording medium using the glasssubstrate with the magnetic resisting head becomes a mainstream from nowon.

In this manner, various improvements are added to the magnetic disc forhigh-density recording, and with the progress of the magnetic disc, newproblems in the glass substrate for the magnetic recording medium ariseone after the other. One of them is high-level cleaning of a glasssubstrate surface. Specifically, foreign particles sticking to the glasssubstrate surface may cause defects of a thin film formed on the glasssubstrate surface, or form convex portions on a thin film surface. Anadequate glide height cannot be obtained.

Moreover, in the regeneration of the magnetic recording medium using theglass substrate with the magnetic resisting head, if the flying heightof the head is lowered to enhance recording density, regeneration errormay occur, or regeneration cannot be performed. Such disadvantageousphenomenon is caused by thermal asperity resulting from the convexportions formed on the magnetic disc surface by the particles on theglass substrate. In this case, heat is generated on the magneticresisting head (the magnetro-resistive head), the resistance of the headis fluctuated, and electromagnetic conversion is adversely affected.

The cause of the foreign particles on the surface of the glass substratefor the magnetic recording medium lies in that a glass substrate endsurface having no smooth state grinds against a wall face of a resincase. Resin or glass particles generated by the grinding, and otherparticles caught by inner and outer peripheral end surfaces of the glasssubstrate stick to the surface. Inventors et al. have found thatespecially the inner peripheral end surface of the glass substrate iscoarser than the outer peripheral end surface, more easily catchesparticles, and obstructs the high-level cleaning of the glass substratesurface.

Additionally, proposed is a technique of removing cracks generated on anend surface portion of a glass substrate by chemical etching to enhancea substrate strength (Japanese Patent Application Laid-open No.230621/1995). In this case, the depth of the crack is decreased, but thecrack is etched, enlarged and indented to easily catch particles, whichfurther obstructs the high-level cleaning of the glass substratesurface. Moreover, in the chemical etching, it is difficult to controlthe surface precision of the end surface portion at a high level.Furthermore, it is difficult to completely remove cracks, and only aninsufficient deflection strength is provided.

SUMMARY OF THE INVENTION

The present invention has been accomplished under the circumstancesabove, and a first object thereof is to provide a polishing method, apolishing device, and the like in which an end surface of a glasssubstrate or the like can efficiently be smoothed at low cost and highlevel, especially, an inner peripheral end surface of the glasssubstrate or the like difficult to be polished can efficiently besmoothed at low cost and high level, so that cleaning of a substratesurface can be achieved to a high level.

A second object is to provide a glass substrate for a magnetic recordingmedium in which cleaning of a glass substrate surface is achieved to ahigh level.

A third object is to provide a magnetic recording medium in whichtroubles caused by foreign particles on a substrate surface areminimized.

According to the first aspect of the invention, there is provided apolishing method in which an inner peripheral end surface and/or anouter peripheral end surface of a circular disc having a circular holein a center portion is polished using an abrasive liquid containing freeabrasive grains.

According to the second aspect of the invention, there is provided apolishing method in which a glass substrate in the shape of a circulardisc having a circular hole in a center portion is immersed in anabrasive liquid containing free abrasive grains, and an inner peripheralend surface and/or an outer peripheral end surface of the glasssubstrate is polished using the abrasive liquid containing the freeabrasive grains.

According to the third aspect of the invention, there is provided apolishing method in which a glass substrate in the shape of a circulardisc having a circular hole in a center portion is immersed in anabrasive liquid containing the free abrasive grains, and an innerperipheral end surface and/or an outer peripheral end surface of theglass substrate is polished using the abrasive liquid containing freeabrasive grains by rotating a polishing brush or a polishing pad incontact with the glass substrate.

In the polishing method, the viscosity of the abrasive liquid containingthe free abrasive grains is preferably 1.5 to 25 cps.

Furthermore, there is disclosed a method of manufacturing a glasssubstrate for a magnetic recording medium which comprises a step ofpolishing an inner peripheral end surface and/or an outer peripheral endsurface of the glass substrate in the polishing method described above.

According to the fourth aspect of the invention, there is provided aglass substrate for a magnetic recording medium in which surfaceroughness Ra of a chamfered portion and/or a side-wall portion of aninner peripheral end surface and/or an outer peripheral end surface is0.001 to 0.04 μm.

The magnetic recording medium can be prepared by forming at least amagnetic layer on the aforementioned glass substrate.

The magnetic recording medium can be used as a magnetic recording mediumfor a magnetic resisting head (a magnetro-resistive head) (MR head) or agiant magnetic resisting head (a giant magnetro-resistive head) (GMRhead).

As the magnetic layer, a magnetic layer including Co and Pt can be used.

According to the fifth aspect of the invention, there is provided apolishing device which comprises an abrasive liquid container in whichan abrasive liquid is contained, a rotary support disposed on a bottomof the abrasive liquid container, a substrate case detachably mounted onthe rotary support for holding a large number of circular discs eachhaving a circular hole in a center portion, and a rotary brush insertedin the circular holes of the circular discs.

Additionally, each of the inner and outer peripheral end surfaces in thepresent invention includes a chamfered portion and a side-wall portion.

According to the first aspect, by polishing the inner and outerperipheral end surfaces of the glass substrate or the like using theabrasive liquid containing the free abrasive grains, the surfaces can besmoothed more efficiently at lower cost and higher level, as comparedwith a polishing method in which a diamond abrasive wheel (fixedabrasive grain) or a chemical etching is used. Especially, the innerperipheral end surface of the glass substrate or the like difficult tobe polished with high precision can efficiently be smoothed at low costand high level. When the diamond abrasive wheel is used, only raisedportions (apexes of protrusions) of the polished surface are scrapedoff, and poor smoothness is provided. In the chemical etching, cracksare etched, enlarged and indented to easily catch particles, whichobstructs the high-level cleaning of the glass substrate surface.Moreover, the glass substrate end surface provides poor smoothness, andforeign particles are generated by frictional grinding or the like.Furthermore, since it is difficult to completely remove the cracks,inferior deflection strength is provided.

According to the second aspect, by immersing and polishing in theabrasive liquid containing the free abrasive grains, a sufficient amountof the abrasive liquid is allowed to exist on the inner peripheral endsurface and/or the outer peripheral end surface. Therefore, polishinginsufficiency or defect because of liquid shortage can be avoided.

According to the third aspect, by combining the immersion into theabrasive liquid containing the free abrasive grains with the brushpolishing or the like, especially the inner peripheral end surface ofthe glass substrate or another surface difficult to be polished withhigh precision can efficiently be smoothed at low cost and high level.Moreover, not only the chamfered portions but also the side-wallportions of the inner and outer peripheral end surfaces of the glasssubstrate can also be smoothed efficiently at low cost and high level.If both the chamfered portion and the side-wall portion are smooth, theeffect of the present invention is further enlarged.

Moreover, in a case where the inner peripheral end surface defining asmall-diameter hole is polished with the rotary brush, in a mode ofcontinuously supplying the abrasive liquid to the rotary brush, theabrasive liquid is scattered by the rotary brush rotating at a highspeed, and the periphery of the rotary brush is brought in a vacuumstate to reject the abrasive liquid. In this manner, the abrasive liquidis insufficiently applied to the polished surface. However, by theimmersion into the abrasive liquid containing the free abrasive grains,polishing insufficiency or defect is prevented from occurring because ofliquid shortage. Moreover, even when a rotary brush having elasticbristles is used, the elasticity of the bristles immersed in theabrasive liquid is moderated by the viscosity resistance or anotherproperty of the abrasive liquid. Since the bristles are prevented fromcolliding against the polished surface unnecessarily strong, apossibility of scratching or damaging the polished surface canremarkably be reduced. Furthermore, for example, by arranging thebristles helically on a rotation shaft, the fluidity of the abrasiveliquid is prompted. Since a fresh abrasive liquid can constantly becirculated/supplied to the polished surface, polishing efficiency,reproducibility and precision can be enhanced.

By setting the viscosity of the abrasive liquid containing the freeabrasive grains in a range of 1.5 to 25 cps (20° C.), polishingefficiency, reproducibility and polishing precision can be enhanced,while the possibility of scratching or damaging the polished surface canremarkably be reduced. In this respect, the viscosity of the abrasiveliquid containing the free abrasive grains is more preferably in a rangeof 1.8 to 5 cps (20° C.). By employing the above-mentioned polishingmethod, the cleaning of the glass substrate surface can be achieved to ahigh level, and the glass substrate for the magnetic recording mediumsuperior in deflection strength can be manufactured.

According to the fourth aspect, by defining the surface roughness of thechamfered portion and/or the side-wall portion of the inner peripheralend surface and/or the outer peripheral end surface in the glasssubstrate for the magnetic recording medium, the cleaning of the glasssubstrate surface can be achieved to a high level, while the glasssubstrate for the magnetic recording medium superior in deflectionstrength can surely be obtained. In this respect, the surface roughnessof the chamfered portion and/or the side-wall portion of the innerperipheral end surface and/or the outer peripheral end surface in theglass substrate for the magnetic recording medium is more preferably setto Ra of 0.001 to 0.03 μm and Rmax of 0.5 μm or less, where Ra isrepresentative of center-line mean roughness, where Rmax is defined as amaximum height representative of a difference between a highest pointand a lowest point, (defined in Japanese Industrial Standard JIS B0601).

As aforementioned, since the surface of the glass substrate for themagnetic recording medium is cleaned to a high level and the deflectionstrength is enhanced, in the magnetic recording medium, foreignparticles attributed to the end surface fail to stick to the glasssubstrate surface. No defect arises on the thin film formed on the glasssubstrate surface, and the glide height can be lowered.

In the present invention, even if the flying height of the head islowered, the regeneration error attributed to thermal asperity or theimpossibility of regeneration is prevented. Therefore, the magneticrecording medium for the magnetic resisting head (the magnetro-resistivehead) (MR head) or the giant magnetic resisting head (themagnetro-resistive head) (GMR head) can be obtained.

Furthermore, the magnetic recording medium superior in magneticcharacteristics can be obtained.

According to the fifth aspect, by the rotary motion of the rotarysupport and the rotary brush, the inner peripheral end surface or thelike can be polished remarkably efficiently. A high-precision,high-efficiency polishing can be performed without damaging the polishedsurface. Moreover, the rotary motion of the rotary brush allows theabrasive liquid to circulate in a tank, and prevents the abrasive frombeing deposited. Additionally, in addition to the rotary brush, meansfor circulating the abrasive liquid may further be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a polishing device according to anembodiment of the present invention.

FIG. 2 is a partially cutaway perspective view of a glass substrate fora magnetic recording medium.

FIG. 3 is a sectional view of a deflection strength tester.

FIG. 4 is a plan view showing a polishing operation of an outerperipheral end surface.

FIG. 5 is a sectional view showing a polishing operation of an innerperipheral end surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Example

FIG. 1 is a sectional view of a polishing device according to anembodiment of the present invention, and FIG. 2 is a perspective view ofa cut glass substrate for a magnetic disc. A case where a polishingmethod and a polishing device according to the present invention areapplied to polishing of an inner peripheral end surface of a magneticdisc glass substrate will be described with reference to the drawings.

First, an example of the polishing device of the present invention willbe described.

In FIG. 1, numeral 1 denotes objects to be polished or magnetic discglass substrates (hereinafter referred to as MD substrate), 2 denotes asubstrate case in which a large number of MD substrates 1 are storedwhile immersed in an abrasive liquid, 3 denotes a rotary support forrotatably fixing/holding the substrate case 2, 4 denotes a rotary brushinserted in a hole defined by inner peripheries of a large number ofstacked MD substrates 1, and 5 denotes an abrasive liquid container forstoring the abrasive liquid.

The substrate case 2 has a mechanism in which by fastening a fasteningcover 22 via a collar 21 from above in an axial direction, the MDsubstrates 1 are held by a friction coefficient among main surfaces ofthe MD substrates 1 without being influenced by rotation of thesubstrate case 2 and the rotary brush 4. Additionally, an abrasiveliquid flowing hole 23 is formed in an appropriate portion of thesubstrate case 2 in such a manner that the abrasive liquid can circulateinside and outside the case.

The rotary support 3 is connected to a rotation shaft 32 of a rotationshaft unit 31 which is hermetically attached to a center portion of abottom plate 51 of the abrasive liquid container 5, and can be rotatedby a rotary drive unit 34 by which the rotation shaft 32 is operated torotate both forward and backward. Additionally, for the rotary driveunit 34, the number of revolutions is variable, and an appropriatenumber of revolutions can be selected in accordance with a polishingpurpose. Moreover, by supplying air via an air supply path 36 from anair supply port 35 which is made in a rotation shaft cover 33 in therotation shaft unit 31, an air seal layer 37 is formed to prevent theabrasive liquid from flowing into the rotation shaft 32. The abrasiveliquid container 5 has a cylindrical side wall 52 hermetically attachedto an outer periphery of the disc-shaped bottom plate 51, and containstherein an abrasive liquid 50.

The rotary brush 4 is connected to a rotation shaft 42 of a rotary driveunit 41, and operated to rotate both forward and backward. In an initialstate, the rotary brush 4 is set in such a manner that a rotation centerposition of the rotary brush 4 coincides with a rotation center of thesubstrate case 2. Moreover, for the rotary brush 4, to regulate contactlengths of bristles 43 onto the MD substrates 1, the pushing against theinner peripheral end surfaces of the MD substrates 1, i.e., the amountof pushing in a direction perpendicular to a brush rotation shaft can beadjusted by a mechanism (not shown) using an air cylinder or the like. Acam mechanism (not shown) allows the rotary brush 4 to push against theinner peripheral end surfaces and simultaneously to reciprocate alongthe brush rotation shaft while swinging.

As shown in FIG. 5, by inserting an end of the rotary brush 4 oppositeto the rotary drive unit 41 into a bearing to which the rotation shaftis fixed, polishing can be performed while the rotation shaft is notdeviated at the time of polishing of the end surfaces. Therefore, ahigh-precision polishing can be performed without any dispersion insurface roughness and size. As the bearing, there can be used a knownbearing such as a bearing, a ball bearing, a roller bearing or a plainbearing. Incidentally, the bearing can also play a role as a guidemember at the time of the insertion of the rotary brush. In this case,the inner diameter of the inlet of the bearing can preferably beincreased so that the rotation shaft of the rotary brush can easily beinserted into the bearing. A plurality of bearings can be disposed, andthe bearing can also be disposed around the rotation shaft on the sideof the rotary drive unit.

As a material of the bristle 43 of the rotary brush 4, a nylon fibercurled in a snaking shape (diameter of 0.1 to 0.3 mm, length of 5 to 10mm) is used, but instead of the nylon fiber, a vinyl chloride fiber, aswine bristle, a piano wire, a stainless fiber, or the like may be used.If a fiber low in hardness or a fiber high in flexibility is used, agrinding force can be prevented from being excessively enlarged byelastic deformation of the bristle, and scratching or damaging caneffectively be prevented. Moreover, since the curled fiber has goodcontact characteristics for an indentation or the like, for example, achamfered portion 1 b of the MD substrate 1 shown in FIG. 2 canefficiently be polished. However, if the polishing efficiency of thechamfered portions 1 b is not made much of, a straight fiber withoutbeing curled may be used. Additionally, if the bristle 43 for use isformed by molding a resin mixed with an abrasive into an abrasivecontaining bristle, polishing rate can further be enhanced.

Cerium oxide is used as the abrasive, but iron oxide, magnesium oxide,zirconium oxide, manganese oxide or another abrasive can be used. Sincean abrasive close to a material of a polished object (MD substrate) ispreferable, cerium oxide is preferable for the glass substrate. If theabrasive is too hard, the end surface of the glass substrate isunpreferably damaged, and if the abrasive is too soft, it isunpreferably impossible to bring the end surface of the glass substrateto a mirror surface. Additionally, the average grain size is preferably1 to 5 μm. With the average grain size of less than 1 μm, the abrasivehas a weak force for grinding the glass substrate, and the tip end ofthe rotary brush 4 frequently abuts on and directly grinds the glasssubstrate end surface. Therefore, it is difficult to control a chamferedconfiguration of the MD substrate, and a portion between the side-wallportion and the chamfered portion of the end surface undesirably sags.If the average grain size exceeds 5 μm, the particle diameter of theabrasive is large, so that the surface becomes coarse and the abrasiveis liable to precipitate on the bottom of the abrasive liquid container,and polishing may be performed without interposing the abrasive betweenthe rotary brush and the glass substrate, which is undesirable because adesired surface roughness cannot be obtained.

An example of a polishing method using the aforementioned polishingdevice will next be described.

First, the rotary brush 4 is retreated from the substrate case 2 by anappropriate amount, and a large number of MD substrates 1 are clamped inthe substrate case 2 by disposing the collars 21 on the top and bottomof the MD substrates 1 and fitting the fastening cover 22. In this case,a core deviation in the hole defined by the inner peripheries of the MDsubstrates 1 is determined by a clearance which is formed by adimensional difference between the inner periphery of the substrate case2 and the outer peripheries of the MD substrates 1. The clearance needsto be regulated in accordance with the operability and the roundness ofthe inner periphery of the substrate case 2, but an adequate clearanceis in a range from a loose fit to an intermediate fit in JIS B 0401(1986).

The substrate case 2 in which a large number of MD substrates 1 are setis mounted on the rotary support 3. The MD substrates 1 have inner andouter peripheries chamfered or processed otherwise before being set.

Subsequently, the rotary brush 4 on the same line as the rotation centerof the substrate case 2 is inserted into the inner peripheral portion ofthe MD substrates 1 as shown in FIG. 1. The stop position of the rotarybrush 4 is determined in such a manner that the MD substrates 1 from thelowermost substrate 1′ to the uppermost substrate 1″ are stacked withina range in which the bristles 43 of the rotary brush 4 are set.

Subsequently, the abrasive liquid container 5 is filled with only anappropriate amount of abrasive liquid 50. With the appropriate amount,the top end surface of the fastening cover 22 of the MD substrates 1 ispositioned slightly below a liquid surface. The amount is appropriatelydetermined in accordance with the polishing purpose. The abrasive liquidmay be filled before or at the same time the rotary brush 4 is insertedin the hole defined by the inner peripheries of the MD substrates 1.

Subsequently, the pushing amount of the rotary brush 4 is adjusted, sothat the bristles 43 of the rotary brush 4 abut on the inner peripheralend surfaces of the MD substrates 1. In a case where the bristles 43 areformed of curled nylon fibers, the pushing amount is adjusted in such amanner that tip ends of the bristles 43 are pushed against the polishedsurfaces of the MD substrates 1 by about 1 to 5 mm.

Additionally, the contact pressure of the brush by the pushing againstthe inner peripheral end surfaces of the MD substrates 1 is preferablyregulated by the mechanism using the air cylinder or the like.Specifically, for elastic bristles, the air pressure of the air cylinderis preferably in a range of 0.05 to 0.1 MPa, while for soft bristles,the air pressure of the air cylinder is preferably in a range of 0.05 to1 MPa.

Subsequently, while the rotary support 3 and the rotary brush 4 arerotated in reverse to each other, polishing is performed. In this case,the preferable number of revolutions of the rotary brush 4 is 1000 to20000 rpm at the time of idling. In the embodiment, the number ofrevolutions of the rotary support 3 is 60 rpm, the number of revolutionsof the rotary brush 4 in the abrasive liquid is 4000 rpm (10000 rpm atthe time of idling), and polishing time is about ten minutes. After apredetermined amount has been polished, the device is stopped, theabrasive liquid 50 is discharged to an amount to which the substratecase 2 can be taken out, and the substrate case 2 is actually taken out.Additionally, when the substrate case 2 is removed, the rotary brush 4needs to be moved to a position where the rotary brush 4 does notinterfere with attachment/detachment of the substrate case 2. Finally,from the taken substrate case 2, the MD substrates 1 are taken in asequence reverse to a sequence in which the MD substrates 1 are set.

Evaluation

The surface roughness of the inner peripheral end surface (the chamferedportion 1 b and/or the side-wall portion 1 a) of the glass substrateobtained in the method described above was Rmax of 0.5 μm and Ra of 0.03μm.

Second Example

A glass substrate for a magnetic recording medium and the magneticrecording medium were manufactured through the following processes.

(1) First Sanding Process

First, glass substrates formed of aluminosilicate glass cut in discshapes being 66 mm and 96 mm in diameter and 1.1 mm and 1.4 mm inthickness from a sheet glass formed in a down drawing process with agrinding wheel were ground and processed with a relatively coarsediamond wheel to mold substrates 95 mm (3.5 inches) and 65 mm (2.5inches) in diameter and 0.8 mm and 0.6 mm in thickness.

In this case, instead of using the down drawing process, molten glassmay directly be pressed using upper, lower and barrel molds to obtain aglass disc.

Additionally, as the aluminosilicate glass used was a chemicalreinforcing glass which mainly contains, in terms of mol %, 57 to 74% ofSiO₂, 0 to 2.8% of ZnO₂, 3 to 15% of Al₂O₃, 7 to 16% of LiO₂, and 4 to14% of Na₂O.

Subsequently, the glass substrate was subjected to a sanding processing.The sanding process has a purpose of enhancing dimensional precision andconfiguration precision. The sanding processing was performed using alapping machine with an abrasive grain size of #400.

Specifically, by using an alumina abrasive grain with a grain size of#400, setting a load L to about 100 kg, and rotating inner and outerrotary gears, opposite surfaces of the glass substrate held in a carrierwere lapped to provide a face precision of 0 to 1 μm and a surfaceroughness Rmax (measured according to JIS B 0601) of about 6 μm.

Subsequently, a circular hole (diameter of 20 mm) was made in a centerportion of the glass substrate using a cylindrical wheel, and outer andinner peripheral end surfaces were subjected to a predetermined chamferprocessing. At the time, the surface roughness of the inner or outerperipheral end surface of the glass substrate was about 14 μm in Rmax.

(2) End Surface Polishing Process

Subsequently, the inner peripheral end surface of the glass substratewas polished using the polishing device and the polishing method shownin the first example.

Additionally, when the end surfaces of the stacked glass substrates arepolished, in order to avoid scratches or the like on the main surface ofthe glass substrate more carefully, the end surface polishing process ispreferably performed before a first polishing process described later,or before and after a second polishing process.

The glass substrate with the end surfaces polished as aforementioned wascleaned with water.

(3) Second Sanding Process

Subsequently, lapping was performed by using the lapping machine and analumina abrasive grain with a grain size of #1000, setting the load L toabout 100 kg, and rotating the inner and outer rotary gears, so that thesurface roughness Rmax of the opposite surfaces of the glass substratewas about 2 μm.

After completing the sanding processing, the glass substrate wassuccessively immersed and cleaned in cleaning tanks of neutral detergentand water.

(4) First Polishing Process

Subsequently, the first polishing process was applied. For a purpose ofremoving residual scratches and deformation after the aforementionedsanding processes, the fist polishing process was performed using apolishing device.

Specifically, the first polishing process was carried out using a hardpolisher (cerium pad MHC15 manufactured by Speed Fan Co., Ltd.) as apolisher (polishing cloth) in the following polishing conditions:

Abrasive liquid: cerium oxide plus water

Load: 300 g/cm² (L=238 kg)

Polishing time: 15 minutes

Removed amount: 30 μm

Number of revolutions of lower platen: 40 rpm

Number of revolutions of upper platen: 35 rpm

Number of revolutions of inner gear: 14 rpm

Number of revolutions of outer gear: 29 rpm

After completing the first polishing process, the glass substrate wassuccessively immersed and cleaned in cleaning tanks of neutraldetergent, pure water, pure water, IPA (isopropyl alcohol), and IPA(steam drying).

(5) Second Polishing Process

Subsequently, the second polishing process was performed by using thepolishing device used in the first polishing process and replacing thehard polisher with a soft polisher (Polylux manufactured by Speed FanCo., Ltd.). The polishing conditions were the same as those in the firstpolishing process except that the load was 100 g/cm², the polishing timewas five minutes and the removed amount was 5 μm.

After completing the second polishing process, the glass substrate wassuccessively immersed and cleaned in cleaning tanks of neutraldetergent, neutral detergent, pure water, pure water, IPA (isopropylalcohol), and IPA (steam drying). Additionally, ultrasonic waves wereapplied to the cleaning tanks.

(6) Chemical Reinforcing Process

After completing the grinding and polishing processes as aforementioned,the glass substrate was subjected to chemical reinforcement.

The chemical reinforcement was performed by mixing potassium nitrate(60%) and sodium nitrate (40%) to prepare a chemical reinforcingsolution, heating the chemical reinforcing solution to 400° C., andimmersing the glass substrate cleaned and preheated to 300° C. in thechemical reinforcing solution for about three hours. In order tochemically reinforce the entire surface of the glass substrate,immersion was performed while a plurality of glass substrates were heldat the end surfaces in a holder.

Through the immersion processing in the chemical reinforcing solution,lithium ions and sodium ions in a glass substrate surface layer weresubstituted for sodium ions and potassium ions in the chemicalreinforcing solution, respectively, to reinforce the glass substrate.

The thickness of a compression stress layer formed on the surface layerof the glass substrate was about 100 to 200 μm.

After the chemical reinforcement was completed, the glass substrate wasimmersed in a water tank of 20° C., quenched and held for about tenminutes.

The quenched glass substrate was immersed and cleaned in concentratedsulfuric acid heated to about 40° C.,

After completing the cleaning in the sulfuric acid, the glass substratewas successively immersed and cleaned in cleaning tanks of pure water,pure water, IPA (isopropyl alcohol), and IPA (steam drying).Additionally, ultrasonic waves were applied to the cleaning tanks.

Evaluation

The surface roughness Ra of the inner peripheral end surface of theglass substrate for the magnetic recording medium obtained through theabove-mentioned processes was 0.028 μm on the chamfered portion 1 b and0.030 μm on the side-wall portion 1 a shown in FIG. 2. Moreover, thesurface roughness Ra of the main surface of the glass substrate was 0.3to 0.7 nm (measured with the interatomic force microscope (AFM)).Observation of the end surface with an electronic microscope (4000times) showed a mirror surface state.

Additionally, no foreign particles or cracks were found on the innerperipheral end surface of the glass substrate for the magnetic recordingmedium, and for the glass surface, no particles causing foreignparticles or thermal asperity were found.

Furthermore, when the deflection strength was measured using adeflection strength tester (Shimazu Autograph DDS-2000) shown in FIG. 3,it was 12 to 20 kg. Additionally, when the deflection strength wasmeasured in the same manner while changing a chemical reinforcing level,it was about 10 to 25 kg.

(7) Magnetic Disc Manufacture Process

A texture layer of sputtered Aluminum nitride (AlN), a Cr underlying(primary) layer, a CrMo underlying (primary) layer, a CoPtCrTa magneticlayer, and a Carbon protective layer were successively formed on each ofopposite surfaces of the glass substrate for the magnetic disc obtainedthrough the processes described above using an in-line type sputteringdevice (apparatus), to obtain an MR head magnetic disc.

For the resultant magnetic disc, it was confirmed that no defects on themagnetic layers or other films were generated by foreign particles.Additionally, a glide test showed neither hit (hit of the head on amagnetic disc surface) nor crush (crush of the head against protrusionson the magnetic disc surface). Furthermore, a regeneration test with themagnetic resisting head (the magnetro-resistive head) showed noregeneration error by the thermal asperity.

Third Example

A glass substrate for a magnetic recording medium and the magneticrecording medium were obtained in the same manner as the second example,except that, as shown in FIG. 4, the outer peripheral end surfaces ofthe glass substrates were polished (sprayed with or immersed in theabrasive liquid) for 30 minutes by rotating the rotary brush 4 with adiameter of 230 mm (bristle length of 10 to 30 mm) at 700 to 1000 rpmand by rotating the stacked MD substrates 1 at 60 rpm as shown in FIG.4, before the inner peripheral end surfaces were polished.

As a result, the surface roughness of the outer peripheral end surfacewas 0.6 μm in Rmax and 0.04 μm in Ra, while the surface roughness of theinner peripheral end surface was 0.5 μm in Rmax and 0.03 μm in Ra.Moreover, when the deflection strength was measured using the deflectionstrength tester (Shimazu Autograph DDS-2000) shown in FIG. 3, it wasabout 18 to 22.5 kg. Additionally, when the deflection strength wasmeasured in the same manner while changing the chemical reinforcinglevel, it was about 10 to 25 kg.

Additionally, in a case where the polishing of the inner and outerperipheral end surfaces was not performed, when the deflection strengthwas measured in the same manner, it was about 5 kg or less. In a casewhere only the outer peripheral end surface was polished, when thedeflection strength was measured in the same manner, it was about 5 to 9kg.

Since the deflection strength indicates almost the same value when theinner and outer peripheral end surfaces are polished and when only theinner peripheral end surface is polished, it can be supposed that thestate of the inner peripheral end surface exerts a strong influence onthe deflection strength. Moreover, it is appreciated that the value ofthe deflection strength can be regulated according to the chemicalreinforcing level.

Fourth Example

A glass substrate for a magnetic recording medium and the magneticrecording medium were obtained in the same manner as the second example,except that a polishing pad was used instead of the rotary brush topolish the inner and outer peripheral end surface.

As a result, the surface roughness Ra of the outer peripheral endsurface was 0.03 μm on the chamfered portion and 0.01 μm on theside-wall portion. Additionally, the surface roughness Ra of the innerperipheral end surface was 0.03 μm on the chamfered portion and 0.01 μmon the side-wall portion.

First Comparative Example

A glass substrate for a magnetic recording medium and the magneticrecording medium were obtained in the same manner as the second example,except that a diamond wheel was used instead of the rotary brush topolish the inner peripheral end surface.

As a result, the surface roughness was in the same degree as the surfaceroughness of the inner and outer peripheral end surfaces immediatelyafter the chamfering processing in the second example. Additionally,when the end surface was observed with the electronic microscope (4000times), a coarse ground state was found and remarkably poor smoothnesswas provided.

Second Comparative Example

A glass substrate for a magnetic recording medium and the magneticrecording medium were obtained in the same manner as the second example,except that the etching processing of the inner and outer peripheral endsurfaces were performed by chemical etching, instead of using the rotarybrush.

As a result, the surface roughness was deteriorated by about 0.1 μm inRa and about 0.7 μm in Rmax as compared with before the etchingprocessing. Additionally, when the end surface was observed with theelectronic microscope (4000 times), cracks were etched, enlarged andindented to easily catch particles. Poor smoothness and residual crackswere found.

Fifth and Sixth Examples

Glass substrates for magnetic discs and the magnetic discs were obtainedin the same manner as the second example, except that soda-lime glass(fifth example) and soda aluminosilicate glass (sixth embodiment) wereused instead of aluminosilicate glass.

As a result, when the soda-lime glass was used, the outer and innerperipheral end surfaces of the glass substrate were slightly rougherthan when aluminosilicate glass was used, which was not a practicalproblem.

Seventh Example

A underlying (primary) layer formed of Al (thickness of 50 angstroms)/Cr(1000 angstroms)/CrMo (100 angstroms), a magnetic layer of CoPtCr (120angstroms)/CrMo (50 angstroms)/CoPtCr (120 angstroms), and a Cr (50angstroms) protective layer were formed on each of the opposite surfacesof the magnetic disc glass substrate obtained in the second exampleusing the in-line type sputtering device (apparatus).

The substrate was immersed in an organic silicon compound solution(mixed liquid of water, IPA and tetraethoxysilane) with fine silicaparticles (particle diameter of 100 angstroms) dispersed therein, andcalcined to form the protective layer formed of SiO₂ having a texturefunction thereon. Furthermore, dipping processing was applied onto theprotective layer using lubricant of perfluoropolyether to form alubricant layer. The magnetic disc for MR head was thus obtained.

For the magnetic disc, the same as in the second example was confirmed.

Eighth Example

A magnetic disc for a thin-film head was obtained in the same manner asthe seventh example, except that the underlying (primary) layer wasAl/Cr/Cr, and the magnetic layer was CoNiCrTa.

Ninth to Twelfth Examples, Third and Fourth Comparative Examples

Subsequently, glass substrates were prepared in the same manner as thefirst example, except that the type or the like of a free abrasive grainwas approximately selected and that the viscosity of the abrasive liquidcontaining the free abrasive grains used in the end surface polishingprocess described above was changed to 1.3 cps (third comparativeexample), 1.5 cps (ninth example), 5 cps (tenth example), 10 cps(eleventh example), 25 cps (twelfth example), and 27 cps (fourthcomparative example). Results are shown in Table. For surface states,results of surface observation by the microscope are shown in circleswhen there are no scratches or damages and in crosses when there arescratches and damages. As shown in the table, scratches and damages areobserved on the end surfaces at the abrasive liquid viscosity of 1.3 cpsor 27 cps. It is supposed that when the viscosity is less than 1.5 cps,portions are generated in which no abrasive grain is interposed betweenthe brush and the glass substrate at the time of polishing, the brushdirectly abuts on the glass substrate, and damages are made. Moreover,when the viscosity exceeds 25 cps, the high viscosity results in anincrease of a load on the rotary drive unit for rotating the rotarybrush, and facilitates coherence of the abrasive. At the time ofpolishing, the abrasive adhering to the abrasive liquid container or thelike is again interposed between the brush and the glass substrate todamage the surfaces.

TABLE THIRD FOURTH COMPAR. NINTH TENTH ELEVENTH TWELFTH COMPAR. EXAMPLEEXAMPLE EXAMPLE EXAMPLE EXAMPLE EXAMPLE VISCOSITY 1.3 cps 1.5 cps 5 cps10 cps 25 cps 27 cps SCRATCH x o o o o x

For the magnetic disc, the same as in the seventh example was confirmed.

The preferred embodiments of the present invention have been describedin the above, but the present invention is not necessarily limited tothe embodiments.

For example, the types of the glass substrate and the magnetic layer arenot limited to those in the embodiments.

Examples of the material of the glass substrate include analuminosilicate glass, a soda-lime glass, a soda aluminosilicate glass,an aluminoborosilicate glass, a borosilicate glass, a quartz glass, achain silicate glass, a glass ceramic such as a crystallized glass, andthe like.

Examples of the aluminosilicate glass include a chemical reinforcingglass which mainly contains 62 to 75 wt % of SiO₂, 5 to 15 wt % ofAl₂O₃, 4 to 10 wt % of Li₂O, 4 to 12 wt % of Na₂O, and 5.5 to 15 wt % ofZrO₂ and which has an Na₂O/ZrO₂ weight ratio of 0.5 to 2.0 and anAl₂O₃/ZrO₂ weight ratio of 0.4 to 2.5.

Moreover, in order to eliminate protrusions on the glass substratesurface caused by insoluble substances of ZrO₂, for example, a chemicalreinforcing glass is preferably used which contains, in terms of mol %,57 to 74% of SiO₂, 0 to 2.8% of ZnO₂, 3 to 15% of Al₂O₃, 7 to 16% ofLiO₂, and 4 to 14% of Na₂O.

By chemically reinforcing the aluminosilicate glass or the like havingthe composition described above, the deflection strength is increased,the compression stress layer is deepened, and a superior Knoop hardnessis provided.

Examples of the magnetic layer include magnetic thin films mainlycomposed of Co such as CoPt, CoCr, CoNi, CoNiCr, CoCrTa, CoPtCr, CoNiPt,CoNiCrPt, CoNiCrTa, CoCrTaPt, CoCrPtSiO, and the like. The magneticlayer may have a multilayered structure (e.g., CoPtCr/CrMo/CoPtCr,CoCrTaPt/CrMo/CoCrTaPt, and the like) which is obtained by dividing themagnetic film with a non-magnetic film (e.g., Cr, CrMo, CrV, and thelike) to reduce noises.

Examples of the magnetic layer for the magnetic resisting head (themagnetro-resistive head (MR head)) or the giant magnetic resisting head(the giant magnetro-resistive head) (GMR head) include a magnetic layerof a Co alloy which contains an impurity element selected from a groupconsisting of Y, Si, rare earth element, Hf, Ge, Sn, and Zn, or an oxideof the impurity element.

Moreover, as the magnetic layer, a granular structure may be used inwhich magnetic particles of Fe, Co, FeCo, CoNiPt, or the like aredispersed in a non-magnetic film composed of ferrite, iron-rare earthelement, SiO₂, BN, and the like. Additionally, the magnetic layer mayhave a recording format of either an inner-face longitudinal type or avertical (perpendicular) type.

The glass substrate for the magnetic recording medium of the presentinvention may be used as a glass substrate for an optical magnetic discwhich rejects microfine particles generated from the glass substrate endsurfaces, or a glass substrate for an electron optical disc like anoptical disc.

Moreover, the polishing method and device of the present invention canbe used as polishing method and device of a glass carbon, a crystalmaterial (including a single crystal material), a brittle material likea ceramic material, a metal material, and the like.

As aforementioned, according to the polishing method and device of thepresent invention, the inner peripheral end surface and/or the outerperipheral end surface of the glass substrate or the like canefficiently be smoothed at low cost and high level. Therefore, thehigh-level cleaning of the substrate surface can be realized, while thedeflection strength can be enhanced.

Especially, by immersing and polishing the inner peripheral end surfaceand/or the outer peripheral end surface in the abrasive liquidcontaining the free abrasive grains, a sufficient amount of abrasiveliquid constantly exists on the end surface, and polishing insufficiencyor defect because of liquid shortage is, therefore, prevented fromoccurring. Moreover, even when elastic bristles are used in the rotarybrush, the elasticity of the bristles immersed in the abrasive liquid ismoderated by the viscosity resistance or another property of theabrasive liquid, and the bristles fail to collide against the polishedsurface unnecessarily strong. Therefore, the possibility of scratchingor damaging the polished surface can remarkably be reduced. Furthermore,for example, by setting the bristles helically on the rotation shaft, afresh abrasive liquid can constantly be circulated and supplied to thepolished surface. Polishing efficiency, reproducibility and precisioncan thus be enhanced.

Moreover, according to the polishing device of the present invention, bythe rotary motion of the rotary support and the rotary brush, the innerperipheral end surface or another surface can be polished remarkablyefficiently. A high-precision, high-efficiency polishing can beperformed without damaging the polished surface.

Furthermore, according to the magnetic recording medium of the presentinvention, the problems caused by the foreign particles on the glasssubstrate surface can be avoided. Especially, a preferable magneticrecording medium for the magnetic resisting head (the magnetro-resistivehead) or the giant magnetic resisting head (the giant magnetro-resistivehead) can be obtained.

What is claimed is:
 1. A polishing device comprising: an abrasive liquidcontainer in which an abrasive liquid is contained; a rotary supportdisposed on a bottom of the abrasive liquid container; a substrate casedetachably mounted on the rotary support for holding a large number ofcircular discs each having a circular hole in a center portion; and arotary brush inserted in the circular holes of the circular discs. 2.The polishing device according to claim 1, further comprising a cammechanism which allows the rotary brush to push against inner peripheralend surfaces of the circular holes and simultaneously to reciprocatealong a brush rotation axis while swinging.
 3. The polishing deviceaccording to claim 1, wherein the rotary brush has opposed ends andincludes a rotation shaft having opposed ends, wherein an end of therotation shaft extends beyond a corresponding end of the rotary brush,one end of the rotation shaft of the rotary brush is attached to arotary drive unit, and a bearing is disposed at the other end of therotation shaft.
 4. The polishing device according to claim 2, whereinthe rotary brush has opposed ends and includes a rotation shaft havingopposed ends, wherein an end of the rotation shaft extends beyond acorresponding end of the rotary brush, one end of the rotation shaft ofthe rotary brush is attached to a rotary drive unit, and a bearing isdisposed at the other and of the rotation shaft.
 5. The polishing deviceaccording to claim 3, wherein an air supply port is disposed at arotation shaft cover for the rotation shaft to supply air to therotation shaft via an air supply path to form an air seal layer aboutthe rotation shaft.
 6. The polishing device according to claim 4,wherein an air supply port is disposed at a rotation shaft cover for therotation shaft to supply air to the rotation shaft via an air supplypath to form an air seal layer about the rotation shaft.